The present invention relates to an electrical termination that facilitates electric resistance fusing of either insulated or uninsulated conductor wire, particularly low melting point wire such as aluminum or aluminum alloy wire.
Conventional techniques have long been known for electrically welding or fusing wire terminations to tang type commutator segments or electrical terminals. In these arrangements a U-shaped tang holds the wire during winding and permits subsequent compression of the tang around the wire between active electrodes. This operation fuses or welds the tang onto the body of the commutator terminal and bonds the wire to the contacting surfaces. At one time, it was customary to mechanically or chemically remove the wire insulation at the location of the weld. However, since the 1950's it has been the practice to fuse or weld the insulated wire and rely on the heat generated during fusing or welding to remove the insulation by flash vaporization.
Although mechanical crimping or soft soldering are prevalent techniques of securing wire terminations, there has been in the last several years a significant increase in the application of fusing and welding techniques for this purpose. Generally, fusing is preferred over welding when joining low resistance metals or when it is desired to hold distortion of the connected parts to a minimum.
The terms fusing and welding as used throughout this patent application have different meanings.
Resistance welding, as used in this patent application, results from passing current through the assembled terminal and wire to internally heat the metal parts to their plastic state and then forging the softened parts together. The electrical resistance of the parts determines the time and current necessary to soften the parts, and this resistance must be higher than the resistance of the welding electrodes. Electrode pressure in this process is such that it will not eliminate all of the surface electrical resistance between the parts and the electrodes. Care is taken to apply only enough pressure to accomplish forging. The nature of the resistance welded joint is that the softened zones of the parts have comingled and amalgamated into an integral bond.
Fusing, on the other hand, as used in this patent application, produces a compression joint in which the parts are held by a surface adhesion contact. In fusing, one and sometimes (in the case of opposed electrodes) both electrodes have a higher electrical resistance(s) than the parts being fused, so that the electrode can act as a heat source to the assembled parts. Power requirements are set so that the electrode-dissipates heat into the assembly to soften the parts without causing them to reach their plastic state. Electrode pressure is applied to thereupon force the softened parts together to form the compression joint as stated. In the compression joint, only about 0.0002 inch of the surface depth is affected with no amalgamation of metals.
Therefore, "fusing" in this patent application means a type of pressure heating that results at the joint primarily in a surface adhesion mechanical bond and not primarily in an amalgamation of molten contacting metals.
For a further detailed description of each of these techniques and the merits and uses of each, see "Commutator Fusing" by Allan Warner, I.E.E.E., Publication Number 69033-EI, Paper Number 71C38 EI-34, Sept. 20, 1971.
In conventional fusing or resistance welding techniques of the type described, the electrical terminal can be formed of any suitable metal such as brass, nickel, nickel alloy, steel, copper, copper alloy, gold, silver or other precious metals and their alloys. Lead wires are normally copper, gold, silver or their alloys. However, difficult problems are encountered when aluminum or aluminum alloy wires are used as part of the assembly. Specifically, unlike the aforementioned suitable metals, the softening points of aluminum and aluminum alloy wire are so low that the necessary heat applied to the assembly to flash vaporize the insulation or sufficiently soften the tang also softens the aluminum wire such that application of even slight pressure to the tang severs or unduly squeezes the softened wire like melted butter. Even if the wire is not severed during fusing, its cross section is so reduced that, upon cooling, it lacks even the minimum strength or cross section for a suitable electrical connection. Because of this characteristic, the use of fusing or resistance welding techniques for aluminum or aluminum alloy wire to tang type terminals or tang type commutators, have been avoided. One attempted solution has been to utilize a specially designed welding or fusing head with a relief therein aligned with the intended wire location. In this way less pressure is applied to the area of the tang aligned with the wire than to the other parts of the tang. However, this solution is neither technically practical nor economical since the fusing electrode is normally made of tungsten (an expensive material) and the edges of the relief wear considerably with use. Also, the specially designed electrode would not have general application and would have to be changed depending upon the nature of the wire material to be connected.
The present invention solves these problems and now facilitates for the first time fusing the electrical terminations of insulated or uninsulated aluminum wire without the risk of unduly weakening or undermining the strength of the wire or the integrity of the termination. Such termination is formed by the use of conventional fusing electrodes.
It is, therefore, a principal object of the invention to provide an electrical terminal that solves the problems and provides the advantages as described above.